1. Field of the Invention
The present invention relates to a bending method and a bending apparatus for glass sheets. In particular, the present invention relates to a bending method and a bending apparatus for glass sheets wherein glass sheets heated to a bending temperature by a heating furnace are conveyed and the glass sheets are bent in a shape along a conveyer path under their own weight.
2. Discussion of the Background
As an apparatus for bending glass sheets during conveyance by rolls, there have been known roll-formed conveyor apparatuses (for instance, U.S. Pat. No. 4,311,509). Such roll-formed conveyor apparatuses include a plurality of conveyer rolls which are bent so as to have arbitrary curvatures. The conveyor rolls are provided at equal distances. The conveyor rolls provide a conveyor path having arbitrary curvatures in the apparatuses. The glass sheets which have been heated to a softening temperature in a heating furnace are conveyed on the conveyor path from an outlet of the heating furnace toward a certain direction. The glass sheets which are conveyed on the conveyor path are bent in a shape along the conveyor path under their own weight during conveyance.
However, the conventional conveyor rolls cause striped defects on a surface of the glass sheets after forming. The striped defects are optical distortion caused during bending the glass sheets, and the distortion degrades the quality of the glass sheets.
The causes of the optical distortion will be explained, referred to FIG. 7. FIG. 7 is a transitional view showing how a glass sheet 1 which has been delivered from an outlet of a heating furnace is conveyed by a plurality of conveyer rolls 2A, 2B, 2C, 2D forming a conveyor path. The glass sheet 1 is bent in a certain shape, being conveyed by the conveyor rolls 2A, 2B, 2C, 2D.
As shown in FIG. 7(B), when the glass sheet 1 has a leading portion 1A contacted with the conveyor roll 2A, the leading portion 1A has heat removed to the conveyor roll 2A. Such removal of heat causes a conveyor roll side of the glass sheet 1 to be thermally shrunk, making the leading portion 1A sag downwardly (C). As shown in (D), the sagged leading portion 1A runs onto the conveyor roll 2B when passing on the conveyor roll 2B. This creates a state that the entire glass sheet 1 is slightly lifted. As shown in (E), when the leading portion 1(A) has passed the conveyor roll 2B, the entire glass sheet 1 returns to a position before being lifted, and contacts with the conveyor rolls 2A, 2B. (F) and (G) are viewes showing a repeat of the operations in (D) and (E). As explained, the glass sheet 1 is conveyed on the conveyor rolls 1A, 2B, 2C, 2D . . . , being slightly vibrated in a vertical direction because of the presence of such sagging motion of the leading portion 1A.
In (C)-(G), attention is directed to arbitrary points a and b on the glass sheet 1 (actually, lines perpendicular to a conveyance direction). The point a rises (D) after contacting with the conveyor roll 2A, and contacts with the conveyor roll 2B after lowering of the glass sheet 1 (E). Next, the point a rises (F) and contacts with the conveyor roll 2C when having lowered (G). In other words, the glass sheet 1 sequentially contacts with the conveyor rolls 2A, 2B, 2C, 2D . . . , during conveyance.
On the other hand, the point b is located above the conveyor roll 2A as shown in (D) when the glass sheet 1 is lifted. After that, the point b is located between the conveyor roll 2A and the conveyor roll 2B in (E) wherein the glass sheet 1 has lowered. This motion prevents contact with the conveyor roll 2A. Repeats of such motion prevent contact with the conveyor rolls 2B, 2C, 2D as well. In other words, the point b does not contact with the conveyor roll 1A, 2B, 2C, 2D . . . at all during conveyance of the glass sheet 1.
When the conventional conveyor rolls are used, portions (e.g. the point a) which are cooled by contact with the conveyor rolls and portions (e.g. the point b) which are not cooled by the conveyor rolls not at all coexist on formed glass sheets. This creates differences among these portions in terms of thermal shrinkage, and the differences are exhibited as striped defects in the formed glass sheets. This is the causes of the optical distortion.